Why is the "300mph barrier" even a thing when we're talking about a French-turned-Italian-turned-German automaker doing test runs in Germany? They only barrier they need to care about is the metric one at 500km/h.
However, 300mph is a valid goal as that was the milestone Sir Malcolm Campbell also barely passed in his final world-record run in the Bluebird.
So relatively muscular imperial miles are correctly used. Readers' eyes might pass over in disinterest should weird & weedy "kilometres" be used instead.
On a more serious note, I'm sure the various manufacturer marketing departments take every opportunity, imperial, metric, Klingon, whatever, to mark a passing of a numeric "barrier" - i.e. a number ending in zeros.